1. Field of the Invention
The present invention relates to a process for the preparation of acetaldehyde. The process comprises the carbonylation of methanol in the presence of hydrogen. In another aspect, this invention relates to a process for the preparation of acetaldehyde which comprises reacting methanol with a mixture of CO and H.sub.2 in the presence of cobalt, a halogen-containing promoter and an ionic iodide.
2. Description of the Prior Art
Acetaldehyde is an intermediate of great value in the chemical industry. It is useful, in particular, for the manufacture of acetic acid and acetic anhydride, e.g., see the Encyclopedia of Chemical Technology, Kirk-Othmer, 3rd Edition, Volume 1, pages 97 et seq.
Industrial processes for producing acetaldehyde are well known to this art. The process which is most widely used at the present time is the direct oxidation of ethylene. Since this hydrocarbon originates from petroleum, however, it is becoming more economical to select raw materials such as methanol which originate from synthesis gas.
The use of methanol as the raw starting material in the synthesis of products which are conventionally derived from ethylene on an industrial scale has formed the subject, and continues to form the subject, of much research. This research essentially relates to the application of carbonylation techniques, i.e., the reaction of carbon monoxide with methanol, optionally in the presence of hydrogen.
Much of the earlier work was related to the homologization of methanol, i.e., the production of ethanol by the carbonylation of methanol.
It is well known that methanol reacts with a mixture of carbon monoxide and hydrogen, e.g., employed in a 1:1 ratio, in the presence of dicobalt octacarbonyl at 185.degree. C. and under a pressure of 360 atmospheres. Under these conditions, a mixture of various products, including ethanol, is obtained with a relatively low selectivity. This reaction is discussed in Wender et al., Science, Volume 113, page 206 (1951).
Other authors have shown that if the homologization reaction is carried out in the presence of cobalt diacetate at 200.degree. C. and under 200 to 350 atmospheres of pressure, the yield of ethanol can be improved by conducting the reaction in the presence of an iodine-containing promoter, e.g., I.sub.2 or CH.sub.3 I, and a gaseous mixture which is rich in carbon monoxide. Such a reaction is discussed in Berty et al., Chem. Tech., Volume 5, pages 260-266 (1956).
Other improvements in the production of ethanol have been provided by adding to the aforenoted catalyst system a phosphorous compound which is soluble in methanol, as in U.S. Pat. No. 3,248,432, by the introduction of ruthenium halides or osmium halides, as in U.S. Pat. No. 3,285,948, or by adding a tertiary phosphine and employing a hydrocarbon as a solvent, as in British Pat. No. 1,546,428.
Nevertheless, these processes cannot be applied on an industrial scale as they do not make it possible to achieve high selectivities with respect to ethanol and they consequently require the use of complex installations for separating the various constituents of the mixture obtained, which adversely affects the overall economy of such a process to an unacceptable extent.
Based upon these findings, other workers have directed their attention towards the production of acetaldehyde. U.S. Pat. No. 3,356,734 discloses that the presence of an amount of cobalt less than 2 millimols of cobalt per mol of methanol in a cobalt-halogen catalyst system favors the conversion of methanol to acetaldehyde. In fact, if the carbonylation of methanol is carried out with a CO/H.sub.2 molar ratio of 1.4 for 2 hours at 185.degree. C. under a pressure of 300 to 400 atmospheres employing the technique disclosed in U.S. Pat. No. 3,356,734, about 130 g of acetaldehyde are obtained per liter of reaction medium per hour. This is a productivity on the order of 70 g of acetaldehyde per hour per gram of cobalt employed in the reaction, taking into account the fact that the dimethoxyethane formed is a potential source of acetaldehyde.
More recent work, e.g., that described in published Japanese Patent Application Nos. 77/136,111 and 77/133,914, has shown that the results achieved with the aid of the cobalt-halogen catalyst system can be substantially improved by adding a large amount of a phosphorus, arsenic, antimony or bismuth compound to the system of cobalt and iodine. Nevertheless, the productivity of acetaldehyde relative to the cobalt employed remains too low for such processes to be of value in industry. Moreover, the known processes convert a substantial part of the methanol to butanol, butanal and butenal. These are products for which there is a variable demand and which adversely affect the overall economy of the process by necessitating additional separation steps.